Laser printer capable of changing a pixel density

ABSTRACT

When a data control portion receives a request for change of a pixel density, a cycle used as a reference cycle for detecting abnormality in a laser beam is changed corresponding to the pixel density. While a rotating speed of a polygon mirror for changing the density of picture element is not constant, emission of the laser beam is forbidden and detection as to abnormality in the laser beam is not effected. A mark for detection of an image density is formed on a photoreceptor where the laser beam is scanned, and outside an area where the image is to be formed. The formation of the detection mark is forbidden while the pixel density is being changed. On the other hand, when the detection mark is being formed, output of an instruction for changing the pixel density is deferred. In addition, emission of the laser beam is forbidden while the pixel density is being changed.

CROSS-REFERENCE TO RELATED, COPENDING APPLICATION

Related, copending applications of particular interest to the instantapplication are U.S. Ser. No. 114,250, entitled "Printing Apparatus",filed Oct. 27, 1987 and U.S. Ser. No. 114,281, entitled "Laser Printer",filed Oct. 27, 1987 and both assigned to the same assignee of theinstant application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printing apparatus, particularly to aprinting apparatus for forming a image using a signal modulated for eachpicture element, such as a laser printer, and more particularly to aprinting apparatus capable of changing a pixel density (a number ofpixels in a unit length).

2. Description of the Prior Art

A laser printer is hereinafter described as an example of a printingapparatus. A laser printer forms an image by a large number of pixelsarranged in a matrix. In the laser printer, a laser beam modulated to beapplied or shut off dependent on information serves to form a latentimage on a photoconductor drum and the image is caused to be visiblethrough development of toner, whereby it is transferred onto plain paperand then fixed. Since a laser printer is capable of modulating a laserbeam at high speed, printing of letters or graphics can be effected withhigh speed and high quality (high density). Accordingly, laser printersare widely utilized as output apparatus of various data processingsystems or picture forming systems using computers.

Images outputted from a host computer or the like have differentdensities and in order to print images adequately upon receipt of thoseoutputs, it is necessary to control a pixel density in a printer in avariable manner according to those outputs. In addition, in order tochange a size of letters to be printed by using a character generatorhaving an identical arrangement of pixels, the pixel density in theprinter needs to be variable.

In order to meet such requirements, laser printers having a variablepixel density have been proposed (for example, Japanese PatentLaying-Open Gazette No. 198076/1984).

However, such a conventional printer is still inconvenient to use.

More specifically, in order to detect abnormality in a laser beam causedby a laser diode or a polygon mirror, the scanned laser beam is detectedby a photo sensor for each scanning and determination is made as towhether an output from the photo sensor is provided in a given period,that is, whether the laser beam passes on the photo sensor in the giventime. However, in the prior art, the period (time) serving as areference for determining the state of the laser beam is fixed andaccordingly abnormality of the laser beam cannot be correctly detectedin a laser printer having a variable scanning period for the laser beam.More specifically, since the period serving as a reference fordetermination needs to be set longer than the longest period (time) ofthe scanning cycles of the laser beam, detection in a critical statecannot be made if a scanning cycle is short and, even in an abnormalcase in which the scanning period is longer than that in a normal state,it happens that the abnormality cannot be detected.

In addition, if a pixel density is to be changed, a period of anunstable scanning cycle exists during a time from the start to the endof the change and since change of the scanning cycle is generallyeffected through mechanical operation, a time as long as several secondsto several tens of seconds is required until the change is completed.Consequently, a conventional laser printer has a disadvantage thatdetention of abnormality occurs in spite of no abnormality in the laserbeam due to unstable setting of the scanning cycle at the time ofchanging the pixel density.

In addition, a laser printer has a process similar to anelectrophotographic process and accordingly a density of an imageprinted finally on plain paper changes dependent on an amount of toner.Therefore, in order to maintain the density of the image in the optimumcondition, it is necessary to control the amount of toner. For thatpurpose, an image of a mark for density detection is formed on aphotoreceptor and after it is developed, it is read by a density reader,so that the density thereof is detected. The mark for density detectionis formed on the photoreceptor at a position where it does not overlapwith an image area of an original image and control is made so that themark is constantly formed at the fixed position.

If the pixel density is changed from the start of a new page of imagessuccessively processed by a host computer or the like, control data fora request for change of the pixel density as well as image data of thesuccessively processed images is outputted from the host computer to thelaser printer. However, if the pixel density is changed while a mark fordensity detection or an image of an image area is being formed, theimage cannot be formed normally and therefore the pixel density must notbe changed until the formation of the images is completed. Consequently,in the prior art, the host computer needs to always observe the state ofthe laser printer and a request for change of the pixel density needs tobe issued in view of timing when the image is not being formed. Thus,signal processing for change of the pixel density on the side of thehost computer is complicated and the processing speed is decreased. Tothe contrary, if the mark for density detection or the image of theimage area is formed while the pixel density is being changed, the imagecannot be formed normally, and, accordingly, the image must not beformed until completion of the changing operation of the pixel density.However, in the prior art, operation for forming the image is startedduring the changing operation of the pixel density and the mark fordensity detection is not formed normally, causing disturbance in controlof the density of the image. To avoid this trouble, the changingoperation of the pixel density must not be performed concurrently withthe formation of the image and for that purpose, the state of the laserprinter should be constantly observed by the host computer and therequest for change of the pixel density needs to be issued in view oftiming not causing any disturbance in formation of the image. As aresult, signal processing for changing the pixel density on the side ofthe host computer is complicated, causing the processing speed to bedecreased.

In the laser printer which has a process similar to anelectrophotographic process as described above, in which a latent imageof pixels is formed on a photoreceptor drum by means of a scanning beam,a beam detector for detecting the scanning beam is provided at aposition in the main scanning direction, ahead of the photoreceptor drumand a laser diode is forced to emit light when the scanning beam passesby the beam detector, whereby a synchronization signal (an SSCAN signal)is obtained. However, if the pixel density (i.e., the printing density)is changed, the scanning speed becomes unstable in a period from thestart to the end of the change and it is difficult to apply correcttiming to forced emission of light of the laser diode during thisperiod. Thus, the forced emission of light cannot be effected at a fixedposition in the scanning direction. In addition, if the pixel density ischanged by change of a rotating speed of the polygon mirror, a time aslong as several seconds to several tens of seconds is required until therotating speed of the polygon mirror becomes stable. Accordingly, duringsuch a long time of the unstable state, the laser beam forcedly emittedis applied to the photoreceptor drum to effect unnecessary exposure,resulting in fatigue of the photoreceptor drum or wasteful consumptionof toner.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a printing apparatuscapable of appropriately detecting abnormality in a laser beam even if apixel density is changed.

The object of the present invention is specifically to provide aprinting apparatus for detecting abnormality in a laser beam based on apredetermined cycle according to the changed pixel density.

The object of the present invention is more specifically to provide aprinting apparatus in which no error occurs in detection of abnormalityin a laser beam when operation for changing a pixel density is beingperformed.

In order to accomplish the above described object, the present inventiongenerally comprises: pixel density change means for changing a pixeldensity of an image to be formed, by changing a scanning cycle ofscanning means; and scanning detection means for detecting a scanningcondition of a laser beam based on a predetermined reference cycleaccording to the changed pixel density.

In addition, in order to accomplish the above described object, inanother aspect of the present invention, the scanning detection meanscomprises density detection means for detecting the changed pixeldensity and detection cycle change means responsive to an output ofdetection from the density detection means for changing the referencecycle for detecting the scanning condition of the laser beam, anddetection by scanning detection means is forbidden while the pixeldensity is being changed by the pixel density change means.

The printing apparatus constructed as described above is able tocorrectly detect abnormality in the laser beam in spite of the change ofthe pixel density because the determination cycle of the detection meansis appropriately changed according to the change of the pixel density.Consequently, abnormality such as defective emission of light of a laserdiode or defective rotation of a polygon mirror can be detectedaccurately. Further, in the above described aspect, unnecessarydetection of abnormality in the laser beam or erroneous detection can beprevented because detection of abnormality in the laser beam isforbidden during operation for changing the pixel density.

Another object of the present invention is to provide a laser printer inwhich formation of a density detection mark or the like is suitablycontrolled irrespective of timing of occurrence of a request for changeof a pixel density.

The above described object of the present invention is specifically toprovide a laser printer in which no disturbance is caused in control ofan image density during operation for changing the pixel density.

The above described object of the present invention is more specificallyto provide a laser printer in which the detection mark or the like isformed normally irrespective of the request for change of the pixeldensity in the case of forming the density detection mark or the like.

In order to accomplish the above described object, the present inventiongenerally comprises: pixel density change means for changing a pixeldensity of an image to be formed, by changing a scanning cycle ofscanning means; detection mark formation means for forming a mark fordetection of a density of the image by applying a laser beam on aphotoreceptor, outside an area where the image is to be formed; imageformation control means for controlling conditions for forming the imageby detecting a density of the formed mark; and mark formation controlmeans for controlling operation of the detection mark formation meansdependent on an operation state of the pixel density change means.

In addition, in order to accomplish the above described object, thepresent invention has another aspect in which: the mark formationcontrol means forbids formation of the mark by the detection markformation means while the pixel density is being changed by the pixeldensity change means, and the laser printer further comprises changeinstructing a means for instructing change of the pixel density and themark formation control means defers output of the instruction from thechange instructing means while the mark is being formed by the detectionmark formation means.

The printer thus constructed makes it possible to prevent disturbance incontrol of the image density caused by abnormal formation of a densitydetection mark, because any image such as the density detection mark isnot formed during operation for changing the pixel density. Further, inthe above described aspect, the density detection mark or the like canbe formed normally irrespective of timing of occurrence of a request forchange of the pixel density because the pixel density is not changeduntil an end of the formation of the density detection mark or the like.

Still another object of the present invention is to provide a printingapparatus in which unnecessary exposure is not applied on aphotoreceptor drum.

The above described object of the present invention is specifically toprovide a printing apparatus capable of suitably controlling a laserbeam during operation for changing the pixel density.

In order to accomplish the above described object, the present inventioncomprises: pixel density change means for changing a pixel density of animage to be formed, by changing a scanning cycle of scanning means; andemission forbidding means for forbidding emission of a laser beam whenthe pixel density is being changed by the pixel density change means.

In the printing apparatus thus constructed, unnecessary exposure is notapplied on the photoreceptor drum because light emission of a laserdiode is stopped during operation for changing the pixel density, andaccordingly, wasteful consumption of toner is not caused.

These objects and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front sectional view of a laser printer of an embodiment ofthe present invention.

FIG. 2 is a perspective view typically representing an optical system ofthe laser printer shown in FIG. 1.

FIG. 3 is a system block diagram for explaining use of the laser printerof the above stated embodiment.

FIG. 4 is an illustration showing signal lines of an interface 201.

FIG. 5 is a block diagram showing details of a printing control portionshown in FIG. 3.

FIG. 6 is an illustration for explaining signals from an output portshown in FIG. 5.

FIG. 7 is an illustration for explaining signals to an input port shownin FIG. 5.

FIG. 8 is a circuit diagram showing an example of a printing datawriting control circuit shown in FIG. 5.

FIGS. 9, 10, 11A, 11B, and 11C are timing charts showing states andtimings of the respective signals of the laser printer of the abovestated embodiment.

FIGS. 12, 13, 14A, 14B, 14C, 15A, 15B, 15C, 16A, 16B, and 17 are flowcharts showing control procedures of the laser printer of the abovestated embodiment.

FIG. 18 is a timing chart showing operation timing of main components ofthe laser printer of the above stated embodiment.

FIG. 19 is an illustration for explaining an image area and a positionof an AIDC (automatic image density control) on a photoconductor in theabove described embodiment.

FIG. 20A is a flow chart showing control procedures of a laser printerof another embodiment of the present invention, which corresponds toFIG. 15A.

FIG. 20B is a flow chart showing control procedures of the laser printerof the above stated embodiment, which corresponds to FIG. 15C.

FIG. 21 is a flow chart showing control procedures of the laser printerof the above stated embodiments, which corresponds to FIG. 16B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a sectional view showing a laser printer 1 and an embodimentof the present invention. A photoconductor 2 has a form of a drum andlatent image is formed on its surface when a laser beam is scannedthereto. In association with the photoconductor 2, there are provided anelectrification charger 3 for uniformly charging the photoconductor 2, adeveloping device 4 for developing a latent image formed by scanning ofa laser beam, a transfer charger 5 for transferring the developed tonerimage onto paper, a separation belt 6 for separating the paper from thephotoconductor 2, a cleaner blade 7 for removing residual toner afterthe transfer, an eraser 8 emitting light to remove electric charge,thereby to attain a uniformly charged state by means of theelectrification charger 3, and a density reader 9 for reading a densityof toner. A paper cassette 10 for containing sheets of copy paper isprovided in a lower portion of the laser printer 1. A paper feed roller11 having a semi-circular section is provided to guide sheets of papercontained in the paper cassette 10 to a transport path. Transportrollers 12 and 13 and resist roller 14 are provided in the transportpath. The registration rollers 14 serve to determine a printing positionfor paper in a sub-scanning direction (which is a directionperpendicular to a direction in which a laser beam scans the drum 2,while the latter direction is referred to as a main scanning direction).Fixing rollers 15 serve to fix the toner transferred by the transfercharger 5. A main body discharge roller 16 is provided to discharge thepaper passing through the fixing rollers 15 to outside the main body. Areversing unit 17 is provided to discharge the paper, directing theprinted surface of the paper downward. The reversing unit 17 comprises apaper guide claw 18 which can be manually operated for selectingdischarge with the printed face directed downward or discharge with theprinted face directed upward, a transport path 19 for discharge with theprinted face directed downward, and discharge rollers 20. There arefurther provided a magnet group 21 and a paper size sensor 22 todetermine a paper size of the sheets in the paper cassette 10. Thisdetermination is made by detection by the paper size sensor 22 as towhether a magnet exists in an area of three bits. The laser printer 1further comprises a paper empty sensor 23 for detecting absence of paperin the paper cassette 10, and paper sensors PS1, PS2 and PS3.

FIG. 2 is a perspective view typically showing an optical system of thelaser printer 1. Referring to FIGS. 1 and 2, a laser diode (referred tohereinafter as LD) 31 is driven by an LD drive portion to be describedafterwards. A collimator lens 32 and a cylindrical lens 33 are providedto correct a spreading range of a laser beam emitted from the LD 31. Apolygon mirror 34 rotates so that a laser beam reflected on the polygonmirror 34 is applied to the photoconductor 2 as a scanning beam 39. Thelaser beam reflected on the polygon mirror 34 passes through a lens 35of fΘ. The f" lens 35 serves to scan the laser beam on thephotoconductor 2 at uniform speed. Reflection mirrors 36 and 37 areprovided to guide the laser beam passing through the fΘ lens 35 to thephotoconductor 2. A beam detector 38 is provided near a side end of thephotoconductor 2 so as to determine a printing position in the mainscanning direction. The optical system in FIG. 2 is adapted so that thescanning beam 39 scans the photoconductor 2 after it passes through thebeam detector 38.

FIG. 3 is a system block diagram for explaining use of the laserprinter 1. In this system, the laser printer 1 is connected to ageneral-purpose data processing apparatus 400 (such as a word processor,a personal computer or other host computer). The laser printer 1comprises a data control portion 300 and a printing control portion 200.

When a printing request is generated in the data processing apparatus400, printer control data for determining a printing operation mode inthe laser printer 1 and printing data for determining printed contentsare transmitted to the data control portion 300 through an interface 301in the form of coded data, because the coded data serve to reducetransmission time. The data control portion 300 receives the coded dataand if the coded data thus received is the printer control data, thedata is transmitted to the printing control portion 200 of the laserprinter 1 through an interface 201 to be described afterwards. On theother hand, if the coded data thus received is the printing data, thecoded data is converted to bit image data and then it is developed in amemory called a bit map memory capable of storing bit image data for onepage. When data for one page is developed, a printing start request isissued to the printing control portion 200 of the laser printer 1through the interface 201. Upon receipt of the printing start request,the laser printer 1 starts a printing operation. In an exposure process,the image data is read out from the bit map memory through the interface201 and the beam from the LD 31 is modulated based on the data, wherebya latent image is formed on the photoconductor 2.

A protocol of the interface 201 and printing control of the laserprinter 1 will be described in the following. The interface 201 servesto communicated data between the data control portion 300 and theprinting control portion 200 in the laser printer 1. It functionallyincludes two interfaces 201a and 201b described below.

Referring to FIG. 5 showing details of the printing control portion 200,a control interface 201a is used to communicate data concerningoperation control of the laser printer 1. More specifically, the datacontrol portion 300 transmits data for designating a print mode of afeed opening or a discharger opening, and data for determining timingfor a printing start request or the like. On the other hand, theprinting control portion 200 transmits data for indicating conditions inthe laser printer 1 such as paper size information or error information,and data for determining timing for completion of printing, discharge ofpaper or the like. Thus interface 201a communicates command data andstatus data. The command data is data for determining timing, while thestatus data is other data. The command data and the status data areshown in Tables 1 and 2.

                  TABLE 1                                                         ______________________________________                                        Commands                                                                      Direction of                                                                  transmission                                                                           Command   Meaning        Answer                                      ______________________________________                                        Data control                                                                           Print     Printing request                                                                             To be given                                 portion  command   for 1 sheet                                                →                                                                      Laser printer                                                                          Printing  Printing density                                                                             To be given                                          density   change request                                                      command   (with charged                                                                 printing density data)                                     Laser printer                                                                          Exposure  Notification of end                                                                          Not to be                                   → end       of exposure    given                                       Data control                                                                           command                                                              portion                                                                       ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Status                                                                        Data control portion                                                                         Laser Printer                                                  Status  Meaning    Status   Meaning                                           ______________________________________                                        Feed    Setting of READY    Ready for printing                                opening feed opening                                                                             PAPER    Nonexistence of paper                                                EMPTY    in cassette                                                          TONER    Less than predetermined                                              EMPTY    amount of toner                                                      JAM      Occurrence of paper jam                                              ERROR    Occurrence of other                                                           errors                                            ______________________________________                                    

An image interface 201b is used to read out image data from the bit mapmemory of the data control portion 300 when a latent image is beingformed on the photoconductor 2, that is, exposure is being effected.

FIG. 4 is an illustration showing signal lines of the image interface201b. S 100 is a write raster signal WRST indicating that exposure isbeing effected. S 101 is a sensor scan signal SSCAN indicating that thescanning beam 30 (as shown in FIG. 2). of the laser beam has passedthrough the beam detector 38. S102 is a data request signal DREQ forrequesting image data of eight bits. S103 is an image data signal ofeight bits outputted in response to the DREQ signal. The WRST signalS100 falls to an L level at the time of exposure, whereby the datacontrol portion 300 is ready for transmitting image data. Start of oneline is detected at the fall of the SSCAN signal S101 and eight-bitparallel data is transmitted to the laser printer in synchronism withthe rise of the DREQ signal S102.

FIG. 5 is a block diagram showing a construction of the printing controlportion 200 of the laser printer 1. The printing control portion 200 hasa so-called multi-chip construction including a CPU 202 as a center, inwhich data can be communicated among the respective chips through a busS10. The CPU 202 is connected with oscillator 203 for generating clockpulses for synchronizing with operation of the CPU 202, and a resetcircuit 204 for resetting the whole circuit when power supply is turnedon. The bus S10 connected to the CPU 202 is connected with a system ROM205 for storing a control program and a system RAM 206 to be anoperation area of the control program. The bus S10 is further connectedwith an output port 207 and an input port 209. The output port 207 isconnected with a drive control portion 208 for driving a motor, asolenoid, a heater and the like. The input port 209 is connected with asensor group 210 including a paper sensor, a density sensor and thelike. The bus S10 is further connected with an operation panel 212having display elements such a LEDs or input elements such as switchingelements through an I/O port 211 for the operation panel.

A scanner drive portion 215 serves to control rotation of the polygonmirror 34. It determines a rotating speed of the polygon mirror 34 inresponse to clock pulses S12 emitted from a timer 213. A value of thetimer 213 can be set in response to an instruction transmitted from theCPU 202 through the bus S10. Consequently, the rotating speed of thepolygon mirror 34 can be changed and set arbitrarily by the CPU 202, sothat a printing density can be changed. The scanner drive portion 215transmits to the input port 209 a polygon lock signal S11 indicatingwhether the polygon mirror 34 rotates at a constant speed or not.

An LD drive portion 218 for controlling drive of the LD 31 modulates abeam of the LD 31 on a signal transmitted from a printing data writecontrol circuit 217. The printing data write control circuit 217prepares modulation data for the LD drive portion 218 based on the imagedata transmitted from the data control portion 300 through the imageinterface 201b, so as to turn on and off the scanning beam 39 at apredetermined position on the photoconductor 2. The printing data writecontrol circuit 217 is connected with the beam detector 38. The printingdata write control circuit 217 is further connected with the bus S10 andthe output port 207. The control interface 201a is controlled by aninterface control circuit 217 connected to the bus S10.

FIG. 6 shows output signals of the output port 207. FIG. 6 onlyrepresents the components to be driven, circuits and connections fordriving those components being omitted from the illustration. Themechanical drive portions such as rollers and a toner supply portion inthis embodiment are all driven by a chain wire driven by the main motor224 and they are turned on and off by means of clutches using solenoids.The output signals transmitted from the output port 207 to the drivecontrol portion 208 include: a signal to a solenoid 220 for determiningwhether drive of the above stated chain wire is to be transmitted to thefeed roller 11 or not; a signal to a solenoid 221 for the registrationrollers 14; a signal to a solenoid 222 for the registration whether aportion for supplying toner to the developing device 4 is to be drivenor not; a signal to an LED 223 attached to the density reader 9; asignal to the main motor 224; a signal to the electrification charger 3;a signal to the transfer charger 5; a signal to a voltage applyingdevice and a high-voltage power supply 227 thereof for applying to thedeveloping device 4 a relative potential with respect to thephotoconductor 2 (referred to hereinafter as a developing bias voltage),to cause toner in the developing device 4 to adhere only to the latentimage formed on the photoconductor 2; a signal to the eraser lamp 8; anda signal to the heater 229. Output signals transmitted to the printingdata write control circuit 217 will be described afterwards.

FIG. 7 shows input signals applied from the sensor group 210 and thescanner drive portion 215 to the input port 209. In FIG. 7, only objectsto be detected by the sensor group 210 are shown and concreteconnections and comparators or the like are omitted from theillustration. The input signals applied from the sensor group 210 to theinput port 209 include: a signal from a switch 230 for detecting openingor closing of a door for separating the laser printer 1 from outside; asignal from a defect detector 231 of the main motor 224; a signal from adefect detector 232 of the electrification charger 3; a signal from adefect detector 233 of the transfer charger 5; a signal from the papersize sensor 22; a signal from a toner sensor 234 for detecting an amountof the toner in the developing device 4; a signal from the paperemptiness sensor 23; signals from paper sensors PS1 to PS3; a signalfrom a density sensor 235 in the density reader 9; a signal from faceup/down switch 235 for detecting a direction of the paper guide claw 18;a signal from an initialization switch portion 237 including twoswitches for setting an initial value of printing density (i.e. pixeldensity) (four different values being able to be set); and a signal froma temperature detector 238 of a heat roller. The temperature detector238 transmits a temperature of the heater to the input port 209.

FIG. 8 is detailed circuit diagram of the printing data write controlcircuit 217.

This circuit 217 serves to determine: a picture printing position in themain scanning direction; a printing portion in the main scanningdirection of a mark for automatic image density control (referred tohereinafter as the AIDC mark); timing for forced emission of light ofthe LD 31 outside an image area generating a synchronizing signal SSCANfor determining the above stated printing position; sampling timing forautomatic power control (referred to hereinafter as APC) of the LD 31.The circuit 217 also serves to detect abnormality in emission of lightof the LD 31 and rotation of the polygon mirror 34. Table 3 shows inputand output signals to and from the circuit 217.

                                      TABLE 3                                     __________________________________________________________________________    From         Input/                                                           or to                                                                              Signal  output                                                                            Function                                                     __________________________________________________________________________    Output                                                                             Start   Input                                                                             Determination of image area position in                      port S114        sub-scanning direction                                       207                                                                                AIDC        Determination of AIDC mark position in                            S108        sub-scanning direction                                            LDON        Forced emission of light of LD 31 for                             S110        starting operation of circuit                                     LD Bias     Determination of existence of bias                                S109        current for power control of LD 31                                DPI SELECT  Determination of fundamental clock                                S113        frequency for printing density                                Reset                                                                              ##STR1##    Establishing of state immediately after                     circuit                                                                            S111        power-on                                                     204                                                                           Beam SSCAN   Output                                                                            Output signal of beam detector 38 as                         detector                                                                           S112        synchronizing signal for writing image                       38                                                                             Data                                                                               ##STR2##                                                                              Output                                                                            A latched signal of START signal by SSCAN                   control                                                                            S100        signal, which notifies exposure state to                     portion          data control portion                                         300                                                                                 ##STR3##    Inversion of SSCAN                                                ##STR4##    Request for 8-bit parallel data                                   ##STR5##                                                                              Input                                                                             8-bit parallel data                                              0˜7 S103                                                           LD drive                                                                           LD DATA Output                                                                            Modulation signal of LD31                                    portion                                                                            S104                                                                     218                                                                                S/H S105    Intensity sampling timing for automatic power                                 control of LD 31 in LD drive portion 218                          LD Bias     Same as S109                                                      S106                                                                     CPU 202                                                                            SSCAN   Output                                                                            Notification of abnormality of polygon                            OUT S107    mirror 34 and LD 31                                          __________________________________________________________________________

Referring to FIG. 8, a clock selector 250 selects from three oscillators251, 252 and 253 clock pulses S115 (referred to hereinafter asfundamental clock pulses) forming a basis for modulation synchronizingclock pulses S119 (referred to hereinafter as image clock pulses) of theLD 31. The selection of the fundamental clock pulses S115 is effected bya DPI SELECT signal S113 from the CPU 202. Thus, a frequency of theimage clock pulses S119 can be selected in response to an instructionfrom the CPU 202, thereby to cause a printing density (i.e., a pixeldensity) of the laser printer 1 to be variable.

In order to change a printing density, it is necessary to change atleast two factors out of the rotating speed of the polygon mirror 34,the modulation frequency of the LD 31 and a feeding speed of paper (i.e.a rotating speed of the photoconductor 2) if no change is made in amechanical construction of the optical system shown in FIG. 2. Thisembodiment adopts a method of changing the rotating speed of the polygonmirror 34 and the modulation frequency of the LD 31. Initialization atthe time of turn-on of power supply is effected according to states ofthe above stated initialization switch portion capable of manualoperation and the initial value is changed thereafter by setting a DPIRQflag a value responding to a change request as described afterwards.Thus, three different printing densities (i.e. pixel densities) can beselected. The three different printing densities are referred tohereinafter as the printing density 1, the printing density 2 and theprinting density 3 according to increase of the density value in thisorder.

Referring now to FIGS. 9, 10 and 11A to 11C, control for determining animage position will be described.

During printing operation, the SSCAN signal S112 is periodicallygenerated from the beam detector 38 as shown in the uppermost portion ofFIGS. 9 and 10. At the rise of the SSCAN signal, a sequential operationsuch as printing in the main scanning direction is started. As shown inFIG. 11A, a Q output (CTGATEO) S116 of a flip-flop 254a rises to an Hlevel at the rise of the SSCAN signal S112 and consequently a Q output(CTGATE1) S117 of a flip-flop 254b rises to the H level in synchronismwith the rise of the fundamental clock pulses (1/1 CLK) S115. At therise of the CTGATE1 signal S117 to the H level, a clear state (CLK) of aflip-flop 255 is cancelled and the flip-flop 255 starts to output 1/2frequency divided clock pulses (1/2 CLK) of the fundamental clock pulsesS115 as a Q output S118. Further, in response to the rise of the CTGATE1signal S117, loading to a four-bit counter 256 (CT1) is cancelled.Accordingly, the four-bit counter 256 starts to count downward inresponse to input of the 1/2 frequency divided clock pulses S118,thereby to provide, as QA, QB, QC and QD outputs, clock pulses obtainedby frequency division of the 1/2 CLK by 1/2, 1/4, 1/8 and 1/16,respectively.

In a start counter 257 (CT2) and an end counter 258 (CT3) fordetermining a start and an end of printing in the main scanningdirection, gates thereof are opened at the rise of the SSCAN signal S112and then counting is started upon receipt of a clock pulse signalobtained by inverting a QD output of the four-bit counter 256 through aninverter I1. Outputs S122 and S123 of the start counter 257 and the endcounter 258, respectively, are at the L level during a countingoperation and they attain the H level when the respective count valuesthereof become zero by downward counting from the set values. The imagearea in the main scanning direction is determined by using the outputsof the start counter 257 and the end counter 258. When counting of theend counter 258 comes to an end, the output S123 (IMEND) rises as shownin FIG. 11C and an L pulse is outputted as a Q output S124 from amonostable multivibrator 259. A Q output of a flip-flop 261 falls to theL level at the rise of the Q output S124. As a result, the LD DATAsignal S104 is forcedly changed to the H level and the LD 31 is causedto emit light.

The forced emission of light of the LD 31 causes again scanning of thebeam detector 38, whereby an H pulse of the SSCAN signal S112 isgenerated. In addition, in response to an output pulse from themonostable multivibrator 259, a borrow (BR) output S138 of the four-bitcounter 256 is supplied as a clear (CLK) signal S140 to the flip-flop254a through a NAND gate G1 and a NOR gate G2. As a result, the Qoutputs S116 and S117 of the flip-flops 254a and 254b respectively,falls to the L level. Accordingly, clock impulses of the Q output S118from the flip-flop 255 are stopped.

The image area in the main scanning direction is determined by means ofthe start counter 257 (CT2) and the end counter 258 (CT3) (as shown inFIG. 19). More specifically, the CPU 202 sets, prior to exposure,suitable values (determined by a paper size) in the start counter 257)for determining a start of an image area at the rise of the SSCAN signaland in the end counter 258 for determining an end of the image area atthe rise of the SSCAN signal S112, whereby the image area is determinedby the outputs S122 and S123 thereof. FIGS. 11B and 11C are detailedtiming charts at points near an end of counting of each of the counters.The DREQ signal S102 and the LOAD signal S131 are supplied in the imagearea as shown in FIG. 10. The data control portion 300 transmitseight-bit parallel data (L DATA) S103 to the control portion 200 inresponse to the rise of the DREQ signal S102. In addition, at the Llevel of the LOAD signal S131, a parallel/serial converter 264 acceptsdata S103 and transmits the data to the LD drive portion 218 as LD drivedata (LD DATA) S104 synchronizing with the image clock pulses (IMCLK)S119.

An image area in the sub-scanning direction is determined by thee,ovs/WRST/ signal S100 obtained by latching the START signal S114 fromthe CPU 202 by the SSCANS signal in the flip-flop 263, as shown in FIG.19. In other word, the DREQ signal S102 is transmitted to the datacontrol portion 300 only when the WRST signal S100 is at the H level.

Now, description will be made of a method of making an AIDC mark. First,a predetermined area in a given position on the photoconductor 2 issubjected to exposure and then to development. Thus, a mark in black isformed in that given position as the AIDC mark. The AIDC mark is usedfor AIDC (automatic image density control), which is effected asfollows. The density of the AIDC mark is read by the reader 9 and if thedensity is lower than a predetermined density, toner is supplied to thedeveloping device 4. The position of the AIDC mark is needless to sayoutside the image area; in this embodiment, the position is in a rangewhere printing is effected in the main scanning direction and outside arange where printing is effected in the sub-scanning direction,corresponding to a position adjacent to the latter range (as shown inFIG. 19). Thus, the AIDC mark is formed in the portion used for printingon the photoconductor 2 and adequate density control can be effected inconsideration of a change in sensitivity due to use of thephotoconductor 2.

The AIDC mark is positioned in the main scanning direction by means ofthe start counter 257 for determining a start of an image area and themonostable multivibrator 260. More specifically, as shown in FIG. 10, anH pulse is provided out of the Q output S125 of the monostablemultivibrator 260 at the rise of the output S122 responsive to an end ofcounting of the start counter 257 having a set value different from theset value for determining the start of the image area, and a mark areacorresponds to a period of the H pulse. On the other hand, the AIDC markis positioned in the sub-scanning direction by cancelling the clear(CLR) state of the monostable multivibrator 260 only at the time ofprinting (as shown in FIG. 9). Since a period of a pulse outputted fromthe monostable multivibrator 260 in response to the AIDC signal S108from the CPU is constant, a width of the mark in the main scanningdirection is changed dependent on the printing density.

Description is now made of occurrence of a SSCANOUT signal. Aprogrammable counter 262 (CT4) provides, as a signal S136 from an OUToutput, an L pulse of a period a little larger than a pulse period ofthe SSCAN signal S112, i.e. a beam scanning period of the beam detector38 in response to the rise of an input signal to an input gate. For thatpurpose, a suitable timer value is set in the CPU 202 according to theprinting density. Since the SSCAN signal S112 is connected to the inputgate, a state of the L level is continued by overlapping with the outputL pulse as far as the polygon mirror 34 rotates at a normal rotatingspeed and the LD 31 normally emits light. However, since the LD 31 doesnot emit light during a period of the L level of an LDBIAS signal, theSSCANOUT signal S107 is forced to be at the L level by means of an ANDgate 265 in that period. The SSCANOUT signal S107 is inputted to aninterruption terminal of the CPU 202.

Referring now to the flow charts in FIGS. 12 to 17 and the timing chartin FIG. 18, control operation of the CPU 202 will be described. First,flags and internal timers used for the control operation will bedescribed.

A PRRJT flag indicates a state in which input of the print command isnot permitted.

A PRNT flag indicates printing operation. If this flag is "1" and theprint command is accepted, printing operation can be started immediatelyafter feeding of paper without requiring preparation for start of themain motor 224 and the photoconductor 2.

A DPIRQ flag indicates a printing density change request as well as aprinting density after the change. The value 0 of the flag indicates norequest and the values 1, 2 and 3 of the flag indicate a request forchange to the printing densities 1, 2 and 3, respectively.

A PLYCH flag indicates that it is necessary to determine whether therotating speed of the polygon mirror 34 becomes constant or not.

An EXPEND flag indicates an end of the exposure.

A BFEXP flag indicates that the print command is accepted and that anexposure process of printing operation is not started.

A DIPAC flag indicates acceptance of a printing density command and acontent of the printing density. The value 0 indicates acceptance of nocommand and the values 1, 2 and 3 indicate acceptance of printingdensity commands having requests for change to the printing densities 1,2 and 3, respectively.

An INHCH flag is set to 1 in a period from a start of exposure to an endof writing. If this flag is set to 1, the printing density cannot bechanged under any condition.

TIM 0 to 14, TIM E0 to E2, TIM S0 to S1 and TIMNX are internal timersfor determining timing for turning on and off the respective componentsduring printing operation.

The characters t1 to t14, tE0 to tE2, tS0 to tS1 and tNX are timervalues as shown in detail in the timing chart of FIG. 18. If the valuet0 is set, counting of a timer is immediately brought to an end.

FIG. 12 is a main flow chart of control operation. When the power supplyis turned on, the RAM 206, the interface 201a, the input port 207, theoutput port 209, the timer 213, the start counter 257 (CT2) and the endcounter 258 (CT3) are initialized. As a result, the timer 213 outputsclock pulses S12 having a period determined by the set value and thestart counter 257 and the end counter 258 maintain the state of the Llevel while counting clock pulses inputted from the outside. Further,the above stated flags and the internal timers are cleared (in the stepN1).

Then, initial start control operation is performed (in the step N2).FIG. 13 is a flow chart showing details of this control operation.First, a heater 229 of the fixing rollers 15 is turned on (in the stepN9) and the value of the initialization switch 237 for the printingdensity is read out (in the step N10). Since the switch portion 237includes two switches, different four states 0, 1, 2 and 3 can beprovided corresponding to the printing densities 1, 2 and 3,respectively. Subsequently, suitable values are set in the timer 213,the DPISELCT signal S113 and the programmable counter 262 (CT4) toobtain the rotating speed of the polygon mirror 34, the frequency of thefundamental clock pulses and the above-mentioned SSCANOUT signalaccording to the selected printing density (in the steps N11, N12 andN13). Thus, if the initialization switch portion 237 has been setaccording to the printing density usually adopted by the user, the valueof the initialization switch portion 237 is read in the step N2 at thetime of turn-on of the power supply and the printing density is set atthe time of initialization corresponding thereto. The printing densitycan be changed thereafter in response to a command from the data controlportion 300 to be described afterwards (in the steps N27 to N35). Thecharacters tc1, tc2 and tc3 shown in the figure are values set in thetimer 213, representing periods of pulses synchronizing with therotating speed of the polygon mirror 34 corresponding to the printingdensities 1, 2 and 3, respectively.

The characters tss1, tss2 and tss3 are values set in the programmablecounters 262 (CT4), representing pulse durations of the "L" pulsegenerated for detection of the SSCANOUT signal corresponding to theprinting densities 1, 2 and 3, respectively.

The heater 229 and the polygon mirror 34 can not be immediately broughtinto a state ready for printing (referred to hereinafter as the READYstate). More specifically, a transition period is required for theheater 229 until a predetermined temperature is attained, and atransition period is also required for the polygon mirror 34 until therotating speed of the polygon mirror 34 becomes a predetermined speed.Accordingly, it is determined in the step N14 whether the heater 229 andthe polygon mirror 34 are both in the READY state. If so, the statusREADY is set to 1 (in the step N15).

When the initial start control operation (in the step N2) is completed,that is, the READY state is established, a main loop is started. In themain loop, communication of status data is at first controlled (in thestep N3). The status data of the data control portion 300 shown in Table2 are read and the status data of the laser printer 1 is transmitted.

Then, command control operation is performed (in the step N4). Morespecifically, this step relates to processing at the time of receivingor transmitting the commands shown in Table 1.

FIGS. 14A to 14C are flow charts showing details of the command controloperation. Particularly, the steps N16 to N27 (shown in FIG. 14A) relateto processing at the time of receiving the print command. When the printcommand is received (in the step N16), it is determined whether an erroroccurs (in the step N17) and whether the apparatus is ready foraccepting the print command based on the PRRJT flag (in the step N18).If an error does not occur and the apparatus is ready for accepting theprint command, the print command is accepted. If the apparatus is notready for accepting the print command, the data NAK is transmitted tothe data control portion 300 (in the step N27). If the print command isaccepted (in the step N19), and if the PRNT flag showing a printingstate is 0, that is, printing is not being effected, the value t0 is setin the timer TIM 0 (in the step N20) and the timers TIME1 and TIME1 arecleared (in the step N21). On the other hand, if the PRNT flag is 1, thevalue t0 is set in the timer TIM5 (in the step N22) and the timer TIME0is cleared (in the step N23). Printing operation is started in eitherthe step N20 or the step N22. When printing operation is started, PRRJTflag is set to 1 and acceptance of the print command is forbidden (inthe step N24). In addition, the BFEXP flag indicating that exposure isnot started is set to 1 (in the step N25) and the data ACK istransmitted to the data control portion 300 (in the step N26).

The steps N28 to N35 (shown in FIG. 14B) relate to processing at thetime of receiving the printing density command.

When the printing density command is received (in the step N28), it isdetermined (in the step N29) whether an error other than a recoverableerror such as emptiness of paper or toner occurs or not. If such anerror occurs, the data NAK is transmitted to the data control portion300 (in the step N35). If the error does not occur, the printing densitycommand is accepted and the value 1, 2 or 3 is set in the DPIAC flagaccording to the request for the printing density (in the steps N31 toN33). Then, the data ACK is transmitted to the data control portion 300(in the step N34).

The steps N36 to N38 (shown in FIG. 14C) relate to processing at thetime of transmitting the exposure end command. If the EXPEND flagshowing an end of exposure is 1 (in the step N36), the exposure endcommand is transmitted to the data control portion 300 (in the step N37)and then the EXPEND flag is cleared (in the step N38). Upon receipt ofthis command, the data control portion 300 prepares for transmission ofthe subsequent printing data. When the command control (in the step N4)is completed, the program proceeds to the sequence control (in the stepN5).

FIGS. 15A to 15C are flow charts showing details of sequence control. Inthis sequence control, successive turn-on and turn-off of the respectivecomponents required for printing operation are controlled bysequentially connecting the internal timers. This control is started byacceptance of the print command in the command control (in the step N4)and the sequential operation is started by setting the value t0 in thetimer TIM0 or TIM5. The timing is shown in detail in the timing chart ofFIG. 18.

When the value t0 is set in the timer TIM0 in the command control (inthe step N4), measuring operation of this timer is immediately broughtto an end in the step N39. Then, the respective components are turned onand off with the timing as shown in FIG. 18 by control procedures in thesteps N39 to N101. On the other hand, if the value t0 is set in thetimer TIM5 in the command control (in the step N4), this timer isimmediately brought to an end in the step N51 and then procedures in thesteps N51 to N101 are executed. The steps N39 to N50 relate topreparatory operation prior to printing operation. In those steps, themain motor 224, the eraser 8, the electrification charger 3 and thedevelopment bias voltage supplier 227 of the developing device 4 areturned on. On the other hand, in response to activation of the LDONsignal and the LDBIAS signal, the LD 31 forcedly emits light. As aresult, the scanning beam 39 is applied to the beam detector 38 and thesequential control in the printing data write control circuit 217 isstarted. The LDON signal becomes non-active after a lapse of asufficient period for starting the above stated control.

The PRNT flag indicating the printing state is immediately set to 1 (inthe step N40) when the timer TIM0 is brought to an end (in the stepN39). This flag is reset to 0 when the sequential printing operation iscompleted.

The steps N51 to N55 relate to control for feeding paper. If a leadingedge of paper passes through the paper sensor PS1 (in the steps N56 andN57, exposure is started after a lapse of a predetermined period (in thestep N58). However, if the polygon mirror 34 is not rotated at constantspeed, that is, the PCYCH flag is 1, exposure is not started and it isdetermined repeatedly whether the PCYCH flag is 0 or not (in the stepN59). The PCYCH flag is changed to 0 (in the step N117) when it isdetermined in the step N116 (in FIG. 16B) that the polygon mirror 34 isrotated at constant speed. When the rotating speed of the polygon mirror34 becomes constant and the PCYCH flag becomes 0, suitable valuesaccording to the printing density and the paper size are set in thestart counter 257 (CT2) and the end counter 258 (CT3) and the STARTsignal S114 is activated to start exposure (in the step N60). Asexposure is started by activation of the START signal S114, the BFEXPflag is reset to 0 and the INHCH flag is set to 1 to forbid change ofthe printing density (in the step N61).

Consequently, if the rotating speed of the polygon mirror 34 is changedprior to exposure upon receipt of the printing density change requestfor example, exposure is not started until it is determined that therotating speed of the polygon mirror 34 becomes constant after thechange.

When the exposure is brought to an end (in the steps N67 to N70), theSTART signal S114 becomes non-active (in the step N68) and the EXPENDflag indicating an end of exposure is set to 1 (in the step N70).

The steps N64 to N66, N71 and N72 relate to control of a registrationroller 14. The roller 14 is turned on (in the step N65) after exposurewith such timing as to enable transfer onto the paper at a predeterminedposition (i.e. after a lapse of a period t10 in this case), and it isturned off (in the step N67) when the paper completely passed throughthe roller 14.

The steps N73 to N86 relate to control for the AIDC mark. If a periodt11 has passed after the exposure, a count value for determining a startposition of the AIDC mark in the main scanning direction is set in thestart counter 257 (in the step N74). The count value in this case isdependent on the printing density. The AIDC signal is immediatelyactivated (in the step N75) and becomes non-activated after a period t12(in the step N78). Thus, in the period t12, the mark is formed at theposition in the main scanning direction determined by the printing datawrite control circuit 217. Thus, this mark is formed, based on the abovestated count value, at the position enabling the density reader 9 toread it. The start counter 257 for determining a start position of animage area is also used to determine the start position of the mark inthe main scanning direction and a special counter or timer is not usedfor formation of this mark. After the mark has been formed, the INHCHflag is immediately reset to 0 and change of the printing density ispermitted. If a printing density change request occurs at this time, thechange of the printing density is started as described later. Inaddition, after a lapse of a period t13 (corresponding to a period inwhich the mark subjected to exposure is developed and arrives at thedensity reader 129) after the mark has been formed, an LED 223 fordensity detection is illuminated (in the step N81) and the density ofthe mark is checked (in the step N82). If the density is lower than apredetermined value, the solenoid 222 for toner supply is turned on (inthe step N83) and after a period t14 it is turned off (in the steps N86and N87.

The steps N88 and N89 relate to control for determining timing foraccepting the subsequent print command. In this embodiment, thesubsequent print command is accepted after a lapse of tNX after start ofexposure (in the step N88) and at that time, the PRRJT flag forbiddingacceptance of the print command is cleared (in the step N89).

The steps N90 to N94 relate to control for determining timing forturning on the transfer charger 5. The transfer charger 5 is turned ononly when paper passes through the transfer charger 5. Thus, thetransfer charger 5 can be prevented from being turned on when the AIDCmark passes through the charger 5, because if the transfer charger 5 isturned on on that occasion, toner would be separated from thephotoconductor 2 and would stain the apparatus.

The step N95 to N102 relate to control for stopping printing operationwhen a printing process is completed and another print request is notissued. When the sequence control (in the step N5) is completed, controlof the image forming portion (in the step N6) is started.

FIGS. 16A to 16B are flow charts showing details of control of the imageforming portion. In those flow charts, the polygon mirror 34, the LD 31and other portions related with image formation are controlled.

In the steps N103 and N108, timing for changing the printing density isdetermined when the printing density command is accepted. In otherwords, although the printing density command is accepted, if it isbefore a start of exposure for the previously accepted print command,the DPIRQ flag indicating the change request is not set (in the stepN103 to N106). In addition, even after the start of the exposure, theprinting density change request by the DPRIQ flag is not accepted untilan end of formation of the AIDC mark, that is, while the INHCH flag isset to 1 (in the steps N107 and N108). Consequently, change of theprinting density is started when all the exposure operations forprinting and the formation of the AIDC mark are completed in response tothe print commands accepted before the printing density command for thechange is accepted.

When the printing density change request is accepted, interruption ofthe SSCANOUT signal is forbidden (in the step N109) and a suitable valuetc1, tc2 or tc3 is set in the timer 213 to obtain the adequate rotatingspeed of the polygon mirror 34 and the fundamental clock frequencyaccording to the requested printing density and the above-mentionedSSCANOUT signal. In addition, a DPISELECT signal is transmitted toselect clock pulses of a suitable oscillator (in the steps N110 toN113), and a suitable timer value tss1, tss2 or tss3 is set in theprogrammable counter 262 (CT4).

Subsequently, the DPIRQ flag is cleared and the PLYCH flag is set to 1to indicate that the rotating speed of the polygon mirror 34 is notconstant (in the step N114). While the PLYCH flag is 1 (in the stepN115), it is determined whether the rotating speed of the polygon mirror34 becomes constant or not (in the step N116). If it becomes constant,the PLYCH flag is cleared (in the step N117) and the forbidding ofinterruption of the SSCANOUT signal is cancelled (in the step N118).

As described above, interruption of the SSCANOUT signal is forbiddenwhile the rotating speed of the polygon mirror 34 is not constant.During that period, the rotating speed of the polygon mirror 34 can notmatch with the fundamental clock frequency and there is a possibilitythat interruption of the SSCANOUT signal will occur although there is noabnormality, if the interruption is not forbidden.

FIG. 17 is a flow chart showing processing when interruption of theSSCANOUT signal occurs. When an interruption occurs, the subsequentinterruption is forbidden (in the step N119) and the power supply of theLD drive portion 118 is turned off (in the step N120), so that the LD 31cannot emit light.

When control of the image forming portion (in the step N6) is completed,error control (in the step N7) is started. In this control, errors suchas emptiness of paper or toner, paper jam, failure of the eraser lamp ordefects in portions subjected to high voltage are detected.

Finally, control procedures other than the printing control, such asdisplay control, temperature control or detection of a paper size, areperformed. Then, the program returns to the step N3, where the abovestated procedures are repeated.

Although a presettable down-counter is used as the timer means in theabove described embodiment to perform the timer function when pulses ofa predetermined period are applied to the counter, other counter ortimer may be used insofar as substantially two kinds of time setting ortime measure operations can be performed to form an image area and amark.

In the above described embodiment, change of the printing density ispermitted in such a manner that the INHCH flag is reset to 0 immediatelyafter the end of formation of the AIDC mark. However, change of theprinting density may be continuously forbidden even after the end offormation of the AIDC mark and it may be permitted after completion of asequence of image formation operations, e.g., after an end of imageformation of a subsequent image area or after an end of image formationof an image area preceding the subsequent AIDC mark.

In the above described embodiment, one AIDC mark is formed with respectto one image and accordingly timing is controlled so that operation froma start in an image area to an end of formation of the AIDC mark may notbe performed concurrently with operation for changing the printingdensity. However, it is needless to say that only the period offormation of the AIDC mark may not overlap with that of the operationfor changing the printing density.

In the following, another embodiment of the present invention will bedescribed.

Since this embodiment is identical to the above described embodiment inmany points, a repetition of the explanation is omitted as for theidentical portions and only differences are hereinafter described.

FIG. 20A is a flow chart showing details of sequence control of a laserprinter of this embodiment of the present invention. This figurecorresponds to FIG. 15A concerning the above described embodiment.

An LDRD flag newly used in this flow chart indicates whether asequential control operation beginning with the SSCAN signal of theprinting data write control circuit 217 is being effected or not. Whenthe LDRD flag is set to 1, it is indicated that the control is beingeffected.

The steps N39 to N50 relate to preparatory operation prior to printingoperation. In those steps, the main motor 224, the eraser 8, theelectrification charger 3 and the development bias voltage supplier 227of the developing device 4 are turned on. On the other hand, in responseto activation of the LDON signal and the LDBIAS signal, the LD31forcedly emits light. As a result, the scanning beam 39 is applied tothe beam detector 38 and the sequential control operation in theprinting data write control circuit 217 is started. The LDON signalbecomes non-active after a lapse of a sufficient period for starting theabove stated control. In this embodiment, the LDRD flag is set to 1simultaneously with the non-activation of the LDON signal (in the stepN46).

The PRNT flag indicating the printing state is immediately set to 1,when the timer TIM0 is brought to an end (in the step N39). This flag isreset to 0 when the sequential printing operation is completed (in thestep N96).

The steps N51 to N55 relate to control for feeding paper, similar tothose in the previously described embodiment. If a leading edge of paperpasses through the paper sensor PS1 (in the steps N56 and N57), exposureis started after a lapse of a predetermined period (in the step N58).However, in this embodiment, if the polygon mirror 34 is not rotated atconstant speed, that is, the PLYCH flag is 1, or if the above statedsequential control operation in the printing data write control circuit217 is not started, that is, the LDRD flag is 0, exposure is not startedand it is determined repeatedly whether the PLYCH flag is 0 or not andwhether the LDRD flag is 1 or not (in the step N59). When the polygonmirror 34 is rotated at constant speed and the PLYCH flag becomes 0, andwhen the above stated sequential control operation in the printing datawrite control circuit 217 is started and the LDRD flag becomes 1,suitable values according to the printing density and the paper size areset in the start counter 257 (CT2) and the end counter 258 (CT3) and thestart signal S114 is activated to start exposure (in the step N60). Asthe exposure is started, the BFEXP flag is reset to 0, and the INHCHflag is set to 1 to forbid change of the printing density (in the stepN61).

FIG. 20B is a flow chart showing the sequence control of the laserprinter of this embodiment. This figure corresponds to FIG. 15Cconcerning the previously described embodiment.

The steps N95 to N101 relate to control for stopping printing operationwhen a printing process is completed and another print request is notissued. When the LDBIAS signal becomes non-active, the sequentialcontrol operation in the printing data write control circuit 217 isstopped and accordingly the LDRD flag is reset to 0 (in the step N102).When the sequence control (in the step N5) is completed, control of theimage forming portion (in the step N6) is started.

FIG. 21 is a flow chart showing details of control of the image formingportion in the laser printer of this embodiment. This figure correspondsto FIG. 16B concerning the previously described embodiment.

When a request for change of a printing density is accepted (DRIRQ ≠ 0in the step N107), the LDBIAS signal becomes non-active and emission ofthe light by the LD31 is stopped. As a result, the sequential controloperation in the printing data write control circuit 217 is stopped andin this embodiment, the LDRD flag is reset to 0 (in the step N109).Subsequently, processing for changing the printing density is executedin the same manner as in the previously described embodiment and whenthe polygon mirror 34 is rotated at constant speed, the PLYCH flag iscleared (in the step N117). Then, in this embodiment, in the same manneras in the case of starting printing, the LDON signal and the LDBIASsignal are activated to cause the LD31 to forcedly emit light, and t1and 52 are set in the timers TIM1 and TIM2, respectively (in the stepN118). As a result, the scanning beam 39 is applied to the beam detector38 and the sequential control operation in the printing data writecontrol circuit 217 is started as shown in FIG. 20A. The LDON signalbecomes non-active after a lapse of a sufficient period for starting theabove stated control, and the LDRD flag becomes 1 (refer to the stepN46).

As described above, the emission of light by the LD31 is stopped whilethe polygon mirror 34 is not rotated at constant speed, because amismatch occurs during this period among the rotating speed of thepolygon mirror 34, the frequency of the fundamental clock pulses and theset values in the start counter 257 (CT2) and the end counter 258 (CT3)(timing for forced emission of light in the main scanning directionbeing determined by those set values). In the case of such a mismatch,forced emission of light could not be applied with normal timing and anunnecessary laser beam would be applied to the photoreceptor 2, causinga fatigue to the photoreceptor 2 or unnecessary consumption of toner.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A laser printer for forming an image on aphotoreceptor through scanning of a laser beam, that can be modulatedbased on image data, by scanning means, and capable of changing a pixeldensity on the photoreceptor, comprising:pixel density change means forchanging the pixel density of the image to be formed, by changing ascanning cycle of said scanning means; scanning detection means fordetecting a scanning condition of said laser beam based on apredetermined reference cycle according to said changed pixel densityand producing a corresponding signal; control means responsive to thescanning detection signal for monitoring the laser printer operation,and means for disabling the effect of the scanning detection means whilethe pixel density is being changed by the pixel density change means toprevent an erroneous response by the control means.
 2. A laser printerin accordance with claim 1, whereinsaid scanning detection meanscomprisesdensity detection means for detecting said changed pixeldensity, detection cycle change means responsive to an output ofdetection from said density detection means, for changing the referencecycle for detecting the scanning condition of said laser beam.
 3. Alaser printer in accordance with claim 1, whereinemission of said laserbeam is forbidden in response to an output of detection from saidscanning detection means indicating abnormality in scanning of saidlaser beam.
 4. A laser printer for forming an image on a photoreceptorthrough scanning of a laser beam, that can be modulated based on imagedata, by scanning means, and capable of changing a pixel density on thephotodetector, comprising:pixel density change means for changing thepixel density of the image to be formed, by changing a scanning cycle ofthe scanning means; scanning detection means for detecting a scanningcondition of the laser beam based on a predetermined reference cycleaccording to said changed pixel density, and producing a correspondingsignal including:density detection means for detecting the changed pixeldensity, and detection cycle change means responsive to an output ofdetection from the density detection means, for changing the referencecycle for detecting the scanning condition of the laser beam, controlmeans responsive to the scanning detection signal for monitoring thelaser printer operation; means for disabling the effect of the scanningdetection means while the pixel density is being changed by the pixeldensity change means to prevent an erroneous response by the controlmeans; detection mark formation means for forming a mark for detectionof a density of the image by applying the laser beam on thephotoreceptor, outside an area where the image is to be formed; imageformation control means for controlling conditions for forming the imageby detecting a density of the formed mark, and mark formation controlmeans for controlling operation of the detection mark formation meansdependent on an operation state of the pixel density change means.
 5. Alaser printer in accordance with claim 1, whereinsaid scanning means isa polygon mirror to be rotated for receiving said laser beam andoutputting the same after reflection thereon, and said pixel densitychange means changes a rotating speed of said polygon mirror.
 6. A laserprinter in accordance with claim 5, whereinforbidding of the detectionby said scanning detection means is cancelled when the changed rotatingspeed of said polygon mirror becomes constant.
 7. A laser printer forforming an image on a photoreceptor through scanning of a laser beammodulated based on image data by scanning means, and being capable ofchanging a pixel density, comprising:pixel density change means forchanging the pixel density of the image to be formed, by changing ascanning cycle of said scanning means, detection mark formation meansfor forming a mark for detection of a density of said image by applyingsaid laser beam on said photoreceptor, outside an area where said imageis to be formed, image formation control means for controllingconditions for forming said image by detecting a density of said formedmark, and mark formation control means for controlling operation of saiddetection mark formation means dependent on an operation state of saidpixel density change means.
 8. A laser printer in accordance with claim7, whereinsaid mark formation control means forbids formation of themark by said detection mark formation means while the pixel density isbeing changed by said pixel density change means.
 9. A laser printer inaccordance with claim 8, further comprisingchange instructing means forinstructing a change of said pixel density, wherein said mark formationcontrol means defers output of an instruction from said changeinstructing means while said mark is being formed by said detection markformation means.
 10. A laser printer in accordance with claim 8,whereinsaid scanning means is a polygon mirror to be rotated forreceiving said laser beam and outputting the same after reflectionthereon, and said pixel density change means changes a rotating speed ofsaid polygon mirror.
 11. A laser printer in accordance with claim 10,whereinsaid mark formation control means forbids emission of said laserbeam while the rotating speed of said polygon mirror is not constant.12. A laser printer in accordance with claim 11, whereinsaid mark isformed by application of said laser beam through rotation of saidpolygon mirror.
 13. A laser printer in accordance with claim 9,whereinsaid scanning means is a polygon mirror to be rotated forreceiving said laser beam and outputting the same after reflectionthereon, and said pixel density change means changes a rotating speed ofsaid polygon mirror.
 14. A laser printer in accordance with claim 13,whereinsaid mark formation control means cancels the deferring of saidoutput of the instruction when the rotating speed of said polygon mirrorbecomes constant.
 15. A laser printer in accordance with claim 14,whereinsaid mark is formed by application of said laser beam throughrotation of said polygon mirror.
 16. A laser printer for forming animage on a photoreceptor through scanning of a laser beam modulatedbased on image data by scanning means, and being capable of changing apixel density, comprising:pixel density change means for changing thepixel density of the image to be formed, by changing a scanning cycle ofsaid scanning means, and emission forbidding means for forbiddingemission of said laser beam while the pixel density is being changed bysaid pixel density change means.
 17. A laser printer in accordance withclaim 16, whereinsaid scanning means is a polygon mirror to be rotatedfor receiving said laser beam and outputting the same after reflectionthereon, and said pixel density change means changes a rotating speed ofsaid polygon mirror.
 18. A laser printer in accordance with claim 17,whereinsaid emission forbidding means forbids emission of said laserbeam while the rotating speed of said polygon mirror is not constant.